Process for producing aluminum alloy for automobile body, and aluminum alloy

ABSTRACT

In the production of an aluminum alloy plate for an automobile body, a molten alloy having an aluminum alloy composition of 3.00% by weight≦Mg≦3.50% by weight, 0.20% by weight≦Fe≦0.60% by weight, 0.10% by weight≦Si≦0.15% by weight, 0.01% by weight≦Ti≦0.05% by weight, Mn≦0.05% by weight and the balance of aluminum containing inevitable impurities is subjected to a liquid metal rolling to provide a plate material; the plate material is subjected to a cold rolling to provide a cold-rolled plate; and the cold-rolled plate is subjected to an annealing treatment at a temperature T set in a range of  340 ° C.≦T≦ 400 ° C. and for a period of time t set in a range of  9  hours≦t≦ 11  hours.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for producing an aluminum alloy plate for an automobile body, and an aluminum alloy having an excellent strength and an excellent ductility, which is used in such producing process.

[0003] 2. Description of the Related Art

[0004] A conventional process for producing an aluminum alloy containing alloy elements such as Mn and Fe sequentially subjects the composition to a liquid metal rolling, a cold rolling and a thermal treatment (see Japanese Patent Application Laid-open No. 10-130767).

[0005] In the liquid metal rolling, the cooling rate of a molten alloy is extremely high, resulting in a large amount of a solid solution of an alloy element in an aluminum matrix. In the conventional aluminum alloy, the manganese (Mn) content is set at a large value, because manganese is an element which enhances strength. For this reason, the amount of a solid solution of manganese in an aluminum matrix is also large. However, manganese (Mn) is disadvantageously crystallized as an Al₆(Mn, Fe)-based intermetallic compound in a subsequent thermal treatment step, which deteriorates the ductility of the resulting aluminum alloy plate.

BRIEF SUMMARY OF THE INVENTION

[0006] The present invention provides a process for producing an aluminum alloy plate having excellent proof strength and excellent elongation for an automobile body, by reducing the manganese content and compensating for the thus-reduced strength with another alloy element.

[0007] According to the present invention, there is provided a process for producing an aluminum alloy plate for an automobile body, which subjects a molten alloy having an aluminum alloy composition of 3.00% by weight≦Mg≦3.50% by weight, 0.20% by weight≦Fe≦0.60% by weight, 0.10% by weight≦Si≦0.15% by weight, 0.01% by weight≦Ti≦0.05% by weight, Mn<0.05% by weight and the balance of aluminum containing inevitable impurities to a liquid metal rolling to provide a plate material, subjects the plate material to a cold rolling to provide a cold-rolled plate, and subjects the cold-rolled plate to an annealing treatment at a temperature T set in a range of 340° C.≦T≦400° C. and for a period of time t set in a range of 9 hours≦t≦11 hours.

[0008] If the manganese content is reduced to an impurity level, i.e., to a value lower than 0.05% by weight in the molten alloy having the aluminum alloy composition, as described above, an aluminum alloy plate containing very little of the Al₆(Mn,Fe)-based intermetallic compound can be produced through the liquid metal rolling, cold rolling and annealing treatment. On the other hand, the reduction in strength of the aluminum alloy plate resulting from the reduction in the Mn content is compensated for by another alloy element, and hence the aluminum alloy acquires an excellent proof strength (0.2% proof strength) and an excellent elongation.

[0009] The reason why the contents of the alloy elements are limited as described above is as follows:

[0010] Magnesium (Mg) exerts the effect of securing strength, while becoming a solid solution in the aluminum matrix to maintain the ductility of the aluminum alloy plate. However, if Mg<3.00% by weight, necessary strength cannot be provided. On the other hand, if Mg>3.50% by weight, proof strength correlated to shape retention is not increased, so that it is difficult to maintain shape in the process of pressing.

[0011] Iron (Fe) has the effect of forming Al—Fe based crystallized grains to increase the strength of the aluminum alloy plate. However, if Fe≦0.20% by weight, necessary strength cannot be provided. On the other hand, if Fe>0.60% by weight, the Al—Fe base crystallized substances become large thereby reducing ductility, resulting in a deteriorated pressability.

[0012] Silicon (Si) exhibits the effect of increasing the strength of the aluminum alloy plate. However, if Si<0.10% by weight, such an effect cannot be provided. On the other hand, if Si>0.15% by weight, the ductility is reduced.

[0013] Titanium (Ti) has the effect of microfabricating re-crystallized substances. However, if Ti<0.01% by weight, the microfabricating effect of titanium cannot be provided. On the other hand, if Ti>0.05% by weight, the pressability of the aluminum alloy plate is deteriorated.

[0014] The Mn content is set at an impurity level. If the Mn>0.05% by weight, the ductility of the aluminum alloy plate is deteriorated due to the crystallization of the Al₆(Mn,Fe)-based intermetallic compound as described above.

[0015] The cold rolling is carried out according to a conventional procedure in order to produce an aluminum alloy plate having a required thickness.

[0016] The lower the proof strength of the aluminum alloy plate, the more excellent the shape retention ability thereof during press forming. However, if the proof strength is too low, there is a fear that the toughness against fracture and breaking is deteriorated. On the other hand, if the proof strength is too high, the spring-back amount during forming is increased, resulting in a deteriorated forming accuracy. Material 5052-O having a proof strength of about 150 MPa and Material 5182-O having a proof strength of about 130 MPa have been conventionally used for an aluminum alloy plate for an automobile body, and considered to be practical materials therefor. It is known that if the variation in proof strength is large, any error in accuracy tends to be large. With these facts in mind, the proof strength σ₂ of the aluminum alloy plate produced according to the present invention is set in a range of 110 MPa≦σ_(0.2)≦140 MPa.

[0017] The elongation of aluminum alloy plate is related to a deformation ability during forming and the toughness of a product. The elongation δ of the aluminum alloy plate produced according to the present invention is set at δ>25% in consideration of the actual performance of an aluminum alloy plate for an automobile body such as the material 5182-O and the like.

[0018] If the temperature T in the annealing treatment of the plate material is high, the plate material is softened to have a high ductility. On the other hand, if the temperature T is low, the aluminum alloy plate tends to have a high strength and a low ductility. The temperature T in the annealing treatment of the plate material may be set at a value of T≧320° C. in order to produce an aluminum alloy plate having an elongation δ≧25%. However, there is a fear that the re-crystallization does not advance at 320° C. in light of the behavior of the proof strength of the plate material having a high Mg content, so that a lower limit of the temperature T in the annealing treatment is set at 340° C. If the temperature T is higher than 340° C., the proof strength σ_(0.2) of the aluminum alloy plate can be suppressed to a value of σ_(0.2)≦140 MPa. On the other hand, in the case where the temperature T in the annealing treatment is higher than 400° C., if the aluminum alloy plate has a large thickness, the proof strength σ_(0.2) of the aluminum alloy plate may be lower than 110 MPa. Therefore, an upper limit of the temperature T is set at 400° C.

[0019] The period of time t for the annealing treatment suffices to be 10 hours, because the temperature T has a margin, but the period of time t is set in the above-described range in consideration of a margin in working.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a graph showing the relationship between the temperature T in an annealing treatment and the proof strength σ_(0.2).

[0021]FIG. 2 is a graph showing the relationship between the temperature T in an annealing treatment and the elongation δ.

[0022]FIG. 3 is a graph showing the relationship between the temperature T in an annealing treatment and the proof strength σ_(0.2).

[0023]FIG. 4 is a graph showing the relationship between the temperature T in an annealing treatment and the elongation δ.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Table 1 shows compositions of Examples (1) to (3) of aluminum alloys. TABLE 1 Aluminum Chemical constituents (% by weight) alloy Mg Fe Si Cu Mn Cr Ti Al Example (1) 3.05 0.45 0.15 0.05 0.01 0.01 0.02 Balance Example (2) 3.40 0.43 0.14 0.04 0.02 0.01 0.03 Balance Example (3) 3.00 0.35 0.15 0.02 0.35 — 0.03 Balance

[0025] Embodiment 1

[0026] (a) Example (1) of an aluminum alloy containing a small amount of magnesium (Mg) was molten to prepare a molten alloy. Then, the molten alloy was fed to a pair of cooling rolls, whereby it was concurrently solidified and rolled. Namely, the molten alloy was subjected to a liquid metal rolling to provide a long plate material having a thickness of 6.0 mm and a width of 1,000 mm.

[0027] (b) Plate pieces cut out from the long plate material were subjected to a cold rolling to provide: a cold-rolled plate having a thickness of 3.0 mm; a cold-rolled plate having a thickness of 2.0 mm; a cold-rolled plate having a thickness of 1.2 mm; a cold-rolled plate having a thickness of 1.0 mm; and a cold-rolled plate having a thickness of 0.8 mm.

[0028] (c) The cold-rolled plates were subjected to an annealing treatment at the temperature varied in a range of 340 to 400° C. and for a period of time t set at 10 hours, to thereby provide aluminum alloy plates for an automobile body.

[0029] (d) A test piece was fabricated from each of the aluminum alloy plates, and subjected to a tensile test to determine a proof strength σ_(0.2) and an elongation δ, thereby providing the results shown in Tables 2 and 3. TABLE 2 Temperature T in Thickness of aluminum alloy plate annealing treatment 3.0 mm 2.0 mm 1.2 mm 1.0 mm 0.8 mm 340° C. 118.5 121.8 126.4 131.0 130.7 360° C. 114.8 116.5 123.8 127.1 127.4 380° C. 112.5 117.0 121.0 125.4 123.0 400° C. 111.9 114.3 118.7 123.6 122.3 Proof strength σ_(0.2) (MPa)

[0030] TABLE 3 Temperature T in Thickness of aluminum alloy plate annealing treatment 3.0 mm 2.0 mm 1.2 mm 1.0 mm 0.8 mm 340° C. 26.4 2.0 26.6 26.8 27.0 360° C. 27.7 27.2 26.7 26.3 26.3 380° C. 27.7 27.1 27.4 27.2 27.3 400° C. 28.6 27.6 27.0 27.1 26.7 Elongation δ (%)

[0031]FIGS. 1 and 2 are graphs based on Tables 2 and 3 and showing the relationship between temperature T in the annealing treatment, proof strength σ_(0.2) and elongation δ. As apparent from FIGS. 1 and 2, it can be seen that the proof strength σ_(0.2) can be in a range of 110 MPa≦σ_(0.2)≦140 MPa and elongation δ can be δ≦25% in aluminum alloy plates having thickness of 0.8 to 3.0 mm by setting the temperature T in the range of 340° C.≦T≦400° C. and the time t at 10 hours in the annealing treatment.

[0032] Embodiment 2

[0033] (a) Example (2) of an aluminum alloy containing a large amount of magnesium (Mg) was molten to prepare a molten alloy. Then, the molten alloy was subjected to the same liquid metal rolling to provide a long plate material having a thickness of 6.0 mm and a width of 1,000 mm.

[0034] (b) Plate pieces cut out from the long plate material were subjected to a cold rolling to provide: a cold-rolled plate having a thickness of 3.0 mm; a cold-rolled plate having a thickness of 2.0 mm; a cold-rolled plate having a thickness of 1.2 mm; a cold-rolled plate having a thickness of 1.0 mm; and a cold-rolled plate having a thickness of 0.8 mm.

[0035] (c) The cold-rolled plates were subjected to an annealing treatment at temperatures varied in a range of 320 to 460° C. and for period of time t set at 10 hours, to thereby provide aluminum alloy plates for an automobile body.

[0036] (d) A test piece was fabricated from each of the aluminum alloy plates and subjected to a tensile test to determine a proof strength σ_(0.2) and an elongation δ, thereby providing the results shown in Tables 4 and 5. TABLE 4 Temperature T in Thickness of aluminum alloy plate hardening treatment 3.0 mm 2.0 mm 1.2 mm 1.0 mm 0.8 mm 320° C. 117.8 125.6 133.0 134.3 136.4 340° C. 118.7 125.9 132.1 132.0 133.8 360° C. 117.3 124.2 129.9 130.3 132.6 380° C. 112.2 120.5 126.5 126.6 128.8 400° C. 110.5 117.2 122.5 122.8 125.7 420° C. 108.1 114.2 118.9 119.0 121.8 440° C. 105.6 111.1 115.4 115.3 117.8 460° C. 103.2 108.1 111.9 111.6 113.8 Proof strength σ_(0.2) (MPa)

[0037] TABLE 5 Temperature T in Thickness of aluminum alloy plate hardening treatment 3.0 mm 2.0 mm 1.2 mm 1.0 mm 0.8 mm 320° C. 27.1 27.6 26.8 26.4 26.9 340° C. 27.2 26.8 26.0 26.2 25.7 360° C. 27.9 27.9 26.4 27.3 25.9 380° C. 28.6 28.2 27.2 28.2 27.3 400° C. 28.6 27.5 27.4 27.1 27.3 420° C. 29.0 27.8 27.2 27.3 27.2 440° C. 29.3 28.1 27.1 27.4 27.0 460° C. 29.7 28.4 26.9 27.6 26.9 Elongation δ (%)

[0038]FIGS. 3 and 4 are graphs based on Tables 2 and 3 and showing the relationship between temperature T in the annealing treatment, proof strength σ_(0.2) and elongation δ.

[0039] As apparent from FIGS. 1 and 2, it can be seen that the proof strength σ_(0.2) can be in a range of 110 MPa≦σ_(0.2)≦140 MPa and elongation δ can be δ≧25% in aluminum alloy plates having thickness of 0.8 to 3.0 mm by setting the temperature T in the range of 340° C.≦T≦400° C. and the time t at 10 hours in the annealing treatment.

[0040] Comparative Example

[0041] (a) Example (3) of an aluminum alloy was molten to prepare a molten alloy. Then, the molten alloy was subjected to the same liquid metal rolling to provide a long plate material having a thickness of 6.0 mm and a width of 1,000 mm.

[0042] (b) The plate material was subjected to a cold rolling to provide a cold-rolled plate having a thickness of 1.0 mm.

[0043] (c) The cold-rolled plate was subjected to an annealing treatment at 370° C. for 10 hours to provide an aluminum alloy plate.

[0044] (d) A test piece was fabricated from the aluminum alloy plate and subjected to a tensile test to measure a proof strength σ_(0.2) and an elongation δ. As a result, it was found that the proof strength σ_(0.2) was 113 MPa which met the requirement of 110 MPa≦σ_(0.2)≦140 MPa, but the elongation δ was 22% which did not meet the requirement of δ≧25%. This is because the Mn content was 0.35% by weight which is out of the requirement of Mn<0.05% by weight.

[0045] Thus, it is possible to provide a process for producing an aluminum alloy plate for an automobile body having excellent proof strength and excellent elongation by employing the process described above.

[0046] It is also possible to provide an aluminum alloy having excellent strength and excellent ductility, which is suitable for producing an aluminum alloy plate for an automobile body having excellent proof strength and excellent elongation.

[0047] The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. 

What is claimed is:
 1. A process for producing an aluminum alloy plate for an automobile body, comprising the steps of: subjecting a molten alloy having an aluminum alloy composition of 3.00% by weight≦Mg≦3.50% by weight, 0.20% by weight≦Fe≦0.60% by weight, 0.10% by weight≦Si≦0.15% by weight, 0.01% by weight≦Ti≦0.05% by weight, Mn<0.05% by weight and the balance of aluminum containing inevitable impurities to a liquid metal rolling to provide a plate material; subjecting said plate material to a cold rolling to provide a cold-rolled plate; and subjecting said cold-rolled plate to an annealing treatment at a temperature T set in a range of 340° C.≦T≦400° C. and for a period of time t set in a range of 9 hours≦t≦11 hours.
 2. An aluminum alloy used for producing an aluminum alloy plate for an automobile body through a liquid metal rolling, a cold rolling and an annealing treatment, said aluminum alloy comprising 3.00% by weight≦Mg≦3.50% by weight, 0.20% by weight≦Fe≦0.60% by weight, 0.10% by weight≦Si≦0.15% by weight, 0.01% by weight≦Ti≦0.05% by weight, Mn<0.05% by weight and the balance of aluminum containing inevitable impurities. 